Assembly line manufacturing systems often require measurement of working material, workpiece, or part produced on the assembly line. One method of measuring a workpiece produced on an assembly line is the manual measurement of the part after the part is manufactured. Each produced workpiece or a sample thereof is taken off the assembly line and manually measured by an operator. However, manually measuring parts and recording the data is time consuming and is subject to measurement and/or data-entry errors. Because of the time and expense required, typically only a small percentage of parts produced are manually measured.
In other systems, the assembly line utilizes limit switches or slide switches to measure a workpiece or a length of workpiece. In such systems, the workpiece moves along the assembly line and when a length of workpiece reaches a limit switch, the switch provides a signal to a controller. The controller may control one or more assembly line production units such as a cut-off press. When the limit switch indicates that the length of material has been achieved, the controller initiates a signal to the cut-off press that cuts the material to the desired workpiece length. Unfortunately, switches of the type are not accurate and are susceptible to variations in workpiece structure, placement, and velocity of the movement of the piece along the assembly line.
Another method is the use of an encoder. Industrial assembly line processes rely on use of a measuring wheel and a rotary encoder to track the movement of material or parts. The linear movement is calculated based on the number of electrical pulses generated per revolution and the circumference of the measuring wheel. The resolution of this measuring system is defined as the linear distance traveled per pulse of the encoder. However, operation systems utilizing an encoder encounter a number of conditions that cause the resolution of the encoder to be inaccurate, thereby resulting in inaccurate measurements. Problems or variations include wear of the wheel due to the wheel being in physical contact with the working material, material buildup on the wheel such as from manufacturing material, cuttings, shavings, dirt, etc. Additionally, poor tracking of the encoder wheel may result due to uneven surfaces on the working material, low friction between the encoder wheel and the working material, or bad alignment of the encoder wheel with the working material.
Two additional sources of length measurement errors are dependent on the type of equipment and type of workpiece or working material being measured. One of these is related to how the encoder wheel tracks on the material. If the encoder is not properly mounted, differences in the thickness of material can cause the encoder wheel to ride on different radii (if the wheel isn't running flat on the part). Because of this and related problems, different materials track differently and must be individually calibrated. Another cause of error is variations in material stretch through a roll-former or similar assembly line equipment. In cases where the encoder is located before the forming process, the stretch of the material must be adjusted for. The material stretch factor may change from coil to coil which requires the operator to make frequent calibrations.
Machine operators frequently attempt to correct the resolution value during operation to provide the desired accurate production and measurement of material from the assembly line. The calibration process is typically straightforward, but can be time-consuming if done properly. In the simplest form, the operator runs a single part and compares the measured length against the programmed length. In practice, an operator obtains a sample of parts produced by the manufacturing process by manually removing them after production and measuring each part. The operator or engineer calculates an average value of the sampled parts. The determined average value is compared against the desired length. The ratio of the desired length to the actual average value is multiplied with the current encoder resolution to determine a corrected resolution.
In the alternative, on length controllers that support it, the measured length is entered into the controller that does the calculations to adjust the effective resolution of the encoder. For best results, the operator should run several long parts, take the average length, and input this into the controller or manually calculate a new correction factor for the encoder resolution. In order to continuously insure accurate production of parts that fall within allowable or desirable tolerances, this manual process must be done on a regular and frequent basis. This process is not only time consuming but is costly both from requiring manual intervention, but also often requires a slow down in the production process. If not done properly or on a regular basis, the production line may produce out-of-spec parts that may be unusable or may not meet production line customer requirements.
Similarly, many assembly lines and operators provide operational data to a statistical process control (SPC) system. Measurement data is manually obtained from a sampled workpiece, recorded and entered into a separate computer system that stores and tracks the data. The SPC system produces statistical reports related to each assembly line or workpiece production. However, such a manual process is costly, time consuming, and is also prone to error.
Therefore, there is a need to provide for an automatic recalculation and updating of the encoder resolution to insure accurate measurement of working material and a workpiece produced from a manufacturing assembly line. There is also a need to eliminate manual measurement of produced parts, by automatically measuring each and every workpiece or part produced on a moving manufacturing assembly line as the workpiece moves along the assembly line, without manual intervention. There is also a need to automatically communicate measurements and data to a remote computer system for storing and for production of reports.